Bending Fatigue Tests on Large Specimens under Sea Water Corrosion
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概要
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The authors have newly designed and manufactured a large rotating bending fatigue test machine (Cantilever type, Max. bending moment 7 t-m, Standard diameter of specimen 125 mmφ) and tested specimens of 135 mm φ under sea water corrosion.<BR>The specimens have been taken from steel bars forged from 1. 4 ton ingots made by acid open-hearth process (forged ratio = 6) and annealed in the furnace. The chemical and mechanical properties of the specimensare as shown below : <BR>Chemical Properties Mechanical Properties<BR>C 0.22 Tensile strength (kg/mm<SUP>2</SUP>) 46.447.0<BR>Si 0.23 Yield point (kg/mm<SUP>2</SUP>) 24.024.5m<BR>Mn 0.42 Elongation (50 mm, %) 36.837.2<BR>P 0.013 Reduction of aea (%) 58.163.2<BR>S 0.043<BR>10 mm φ specimens of the same material having simillar shape have been also tested under sea water corrosion for comparison.<BR>The following facts have been revealed by the tests : <BR>(1) Fatigue strengths of 130 mm φ and 10 mm φ specimens at 10<SUP>8</SUP> stress reversal were 11.2kg/mm<SUP>2</SUP> and 3. kg/mm<SUP>2</SUP> respectively.<BR>(2) Depth of cracks found on the corroded surface of specimens is calculated by the following formula : <BR><I>l</I>=C×log<SUB>10</SUB>(N/Nc)<BR>where <I>l</I> : Depth of corrosion cracks (mm) <BR>C : Constant, determined by the method of corrosion (=0.86 in this experiment) <BR>Nc : Limit of stress reversal at which corrosion cracks will originate (=1.7×10<SUP>6</SUP> in this experiment) <BR>N : No. of stress reversals.<BR>(3) When N does not exceed 1.7×10<SUP>6</SUP>, cracks will not originate.<BR>(4) A reversal stress limit (σ<SUB><I>c</I></SUB>) at which surface cracks have originated on large corroded specimens may be pressumed about 5kg/mm<SUP>2</SUP>.<BR>(5) There is also a reversal stress limit (σ<SUB>0</SUB>) at which fine cracks on the non-corroded specimens begin to develope, σ<SUB>0</SUB> is assumed about 11 kg/mm<SUP>2</SUP> for the steel tested<BR>(6) Failure of corroded specimens may occur when reversal stress has increased as the result of reduction of effective area by surface cracks and exceeds σ<SUB>0</SUB>.<BR>(7) Fatigue limit of large corroded specimens is designated by σ<SUB>0</SUB>.<BR>(8) There may be no fatigue limit for small corroded specimens.
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